This is my first post here, so I hope I'm doing this right. I have been leafing through Genki and making slow progress. At one point it introduces "He said" or "she said" as "to itte imashita." When Genki does it, they already have the person as the subject of the discussion. So the discussion starts out talking about Mary, and then goes "she says". But what if there wasn't already somebody that was being talked about. What if I just wanted to say, "Mary says, 'the dog is white.'" How would this sentence go, complete with particles?

There's several ways to say it, but
「あの犬は白です」とマリーさんが言っています。
is a pretty straightforward translation.

It seems like a little bit of a strange sentence though, both in English and in Japanese.
It makes it sound like Mary has habit of saying the exact phrase 'the dog is white'. (Not that anyone would actually think that, but it's that kind of pattern, and since that's -not- what's being said it fits a bit awkwardly.)

In the case of stating a fact in both languages I'd expect a flat past tense:
Mary said, "The dog is white."
「あの犬は白です」とマリーさんが言いました

Or an indirect quote, which would be like,
Mary says the dog is white.
あの犬が白だとマリーさんが言っています